1. Field of the Invention
The invention relates to radiation-curable oligomers, radiation-curable compositions, coated optical glass fibers, and ribbon assemblies.
2. Description of Related Art
Optical glass fibers are generally coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which is in direct contact with the glass is called the inner primary coating and the overlaying coating(s) is called the outer primary coating.
The inner primary coating is usually a relatively soft coating providing environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending. Microbending in the coated fiber can lead to attenuation of the signal transmission capability of the coated fiber and is therefore undesirable. The outer primary coating(s), which is on the exposed surface of the coated fiber, is typically a relatively harder coating designed to provide a desired resistance to physical handling forces, such as those encountered when the fiber is cabled.
For the purpose of multi-channel transmission, optical glass fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical glass fiber assemblies include ribbon assemblies and cables. A typical ribbon assembly is made by bonding together a plurality of parallel oriented, individually coated optical glass fibers with a matrix material. The matrix material has the function of holding the individual optical glass fibers in alignment and protecting the same during handling and the installation environment. Often, the fibers are arranged in ribbon structures, having a generally flat, strand like structure containing generally from about 2 to 24 fibers. Depending upon the application, a plurality of resulting ribbon assemblies can be combined into a cable which has from several up to about one thousand individually coated optical glass fibers. An example of a ribbon assembly is described in published European patent application No. 194891. In general, a plurality of ribbon assemblies may then be combined together in a cable, as disclosed in U.S. Pat. No. 4,906,067.
Optical glass fiber assemblies provide a modular design which simplifies the construction, installation and maintenance of optical glass fibers by eliminating the need to handle individual optical glass fibers.
Coated optical glass fibers for use in optical glass fiber assemblies are usually coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to facilitate identification of the individual coated optical glass fibers. Thus, the matrix material which binds the coated optical glass fibers together contacts the outer ink layer if present, or the colored outer primary coating.
When a single optical glass fiber of the assembly is to be fusion connected with another optical glass fiber or with a connector, an end part of the matrix layer can be removed to separate each of the optical glass fibers. Desirably, the primary coatings on the coated optical glass fibers, and the ink coating if present, are removed simultaneously with the matrix material to provide bare portions on the surface of the optical glass fibers (hereinafter referred to as xe2x80x9cribbon strippingxe2x80x9d). In ribbon stripping, the matrix material, primary coatings, and ink coating, are desirably removed as a cohesive unit to provide a clean, bare optical glass fiber which is substantially free of residue. This residue can interfere with the optical glass fiber ribbon mass fusion splicing operation, and therefore usually must be removed by wiping prior to splicing. The step of removing the residue can cause abrasion sites on the bare optical glass fiber, thus compromising the strength of the connection. Furthermore, these abrasion sites can lead to unwanted microbending. The ability of ribbon assemblies to provide clean, residue-free, bare optical glass fibers during ribbon stripping is highly desired.
A common method for practicing ribbon stripping at a terminus of the ribbon assembly is to use a heated stripping tool. Such a tool consists of two plates provided with heating means for heating the plates to about 90 to about 120xc2x0 C. An end section of the ribbon assembly is pinched between the two heated plates and the heat of the tool softens the matrix material and the primary coatings on the individual optical glass fiber. The heat-softened matrix material and heat-softened primary coatings present on the individual optical glass fibers can then be removed to provide bare optical glass fiber ends, at which the fusion connections can be made. A knife cut is often used to initiate a break in the matrix material to the inner primary coating. Typically, only about 1 cm to about 4 cm section of the matrix material and coatings on the optical glass fibers need be removed so that identification of the bare individual optical glass fibers can be made by tracing back along the bare optical fiber until the ink coating or colored outer primary coating is seen.
U.S. Pat. No. 5,373,578 discloses a ribbon assembly containing a plurality of coated optical glass fibers. Each of the optical glass fibers is coated with an inner primary coating which is adjacent to the optical glass fiber, with an outer primary coating and ink coating on the outer primary coating. The inner primary coating is modified so that adhesion between the inner primary coating and the optical glass fiber is reduced. This reduction in adhesion facilitates easy removal of the heat-softened primary coating when using a heat stripping method. While this patent discloses, at column 5, lines 10-13, that the adhesion between the inner primary coating and the optical glass fiber should be sufficient to prevent delamination of the inner primary coating from the optical glass fiber, any reduction in the adhesion between the inner primary coating and the optical glass fiber increases the possibility of such undesirable delamination in the presence of moisture. Delamination of the inner primary coating from the optical glass fiber can lead to strength degradation of the optical glass fiber and attenuation of the signal transmitted through the optical glass fiber.
Published European patent application 0262340 discloses a ribbon cable having a xe2x80x9cpeel layerxe2x80x9d as the outermost coating layer on each of optical glass fibers contained within the ribbon cable. During ribbon stripping, the peel layer is destroyed and the matrix material is removed from the coated optical glass fibers. However, after ribbon stripping, the optical glass fibers are still coated with the primary coatings. The primary coatings are not simultaneously removed with the matrix material in the ribbon assemblies disclosed in this publication.
U.S. Pat. No. 5,011,260 discloses a ribbon cable having a xe2x80x9cdecoupling layerxe2x80x9d disposed between the coated optical glass fibers and the matrix material. In this manner, the matrix material may be easily removed from the coated optical glass fibers by application of low stripping force. This patent discloses a general statement that the coatings on the optical glass fiber can be simultaneously removed with the matrix material during ribbon stripping. However, this patent does not teach how to solve the problems associated with residue remaining on the bare optical glass fibers after ribbon stripping conventional ribbon assemblies.
Published European patent application 0407004 discloses a ribbon cable containing a matrix material having sufficient adhesion to the ink coated optical glass fibers to remain thereto during normal use but is easily strippable there from without damaging the integrity of the ink layer on the coated optical glass fibers. Thus, the ribbon assembly disclosed in this publication does not have the capability of removing the primary coatings on the optical glass fibers simultaneously with the matrix material during ribbon stripping, and to provide residue-free bare optical glass fibers.
Published European patent application 0527266 discloses a ribbon cable containing a lubricating xe2x80x9cinterfacial layerxe2x80x9d which separates the matrix material from the coated optical glass fibers. The interfacial layer facilitates easy removal of the matrix material from the coated optical glass fibers. While this publication discloses at page 3, line 15, that the buffer layer and first protective coating can be stripped in one step, there is no disclosure teaching how to accomplish such. Furthermore, the lubricating interfacial layer will inhibit simultaneously removal of the first protective coating with the matrix material. Thus, this publication does not teach how to make a ribbon assembly having the capability of removing the primary coatings on the optical glass fibers simultaneously with the matrix material during ribbon stripping, and to provide residue-free bare optical glass fibers.
U.S. Pat. No. 4,900,126 discloses a ribbon cable in which the bonding between the ink layer and the primary coatings on the optical glass fibers is greater than the bonding between the ink layer and the matrix material. In this manner, the matrix material can be easily removed from the ink coated optical glass fibers without removing the ink layer. This patent does not address the problems associated with removing the primary coating layers simultaneously with the matrix material.
U.S. Pat. No. 4,660,927 teaches a silicone-coated optical fiber in which the soft silicone coating is easily peeled from the surface of the optical glass fibers by finger pressure. The coating contains a first siloxane component having aliphatic unsaturated groups and a second siloxane component having mercaptoalkyl groups. Because the coating is easily peelable by rubbing with finger pressure, the coating has insufficient adhesion to the surface of the optical glass fibers to prevent delamination during most uses. Furthermore, this patent does not address the problems of ribbon stripping, but rather only stripping of a single optical glass fiber. It is generally known that three coating systems (inner primary coating, outer primary coating, and ink coating) having acceptable single fiber strippability possess dramatically different levels of strippability in ribbon form.
U.S. Pat. No. 4,496,210 provides a radiation-curable optical fiber coating composition containing a polysiloxane. However, this patent does not address the problems associated with ribbon stripping.
Japanese Patent Application H3-35210 teaches to combine a liquid lubricant, such as liquid silicone oil or liquid aliphatic oil, with a mercaptosilane compound in an inner primary coating composition. During stripping, when the bond between the surface of the optical glass fiber and inner primary coating is broken the liquid lubricant invades the boundary between the surface of the optical glass fiber and the inner primary coating. The liquid lubricant must not have a high compatibility with the inner primary coating or it will not bleed out of the inner primary coating during stripping. However, the use of a liquid silicone oil or liquid aliphatic oil can reduce the stability of the inner primary coating. Furthermore, since the liquid silicone oil and liquid aliphatic oil are not bound within the inner primary coating, they can be extracted from the inner primary coating.
There are many test methods to determine the performance of a ribbon assembly during ribbon stripping. An example of a suitable test method for determining the stripping performance of a ribbon is disclosed in the article by Mills, G., xe2x80x9cTesting of 4- and 8-fiber ribbon strippabilityxe2x80x9d, 472 International Wire and Cable Symposium Proceedings (1992), the complete disclosure of which is incorporated herein by reference.
Many attempts have been made to understand the problems associated with ribbon stripping and to find a solution to increase ribbon stripping performance. The following publications attempt to explain and solve the problems associated with ribbon stripping: K. W. Jackson, et. al., xe2x80x9cThe Effect of Fiber Ribbon Component Materials on Mechanical and Environmental Performancexe2x80x9d, 28 International Wire and Symposium Proceedings (1993); H. C. Chandon, et. al., xe2x80x9cFiber Protective Design for Evolving Telecommunication Applicationsxe2x80x9d, International Wire and Symposium Proceedings (1992); J. R. Toler, et. al., xe2x80x9cFactors Affecting Mechanical Stripping of Polymer Coatings From Optical Fibersxe2x80x9d, International Wire and Cable Symposium Proceedings (1989); and W. Griffioen, xe2x80x9cStrippability of Optical Fibersxe2x80x9d, EFOC and N, Eleventh Annual Conference, Hague (1993).
There is a need for a radiation-curable, inner primary coating composition which when suitably cured provides the property of releasing from the surface of an optical glass fiber to provide a clean, bare optical glass fiber, subsequent to ribbon stripping.
It is an objective of the present invention to provide novel radiation-curable oligomers which when utilized to formulate inner primary coatings on coated optical glass fibers for use in ribbon assemblies, provides the inner primary coating with the inherent property to release as a coherent layer from an optical glass fiber during ribbon stripping to provide a clean, bare optical glass fiber.
Another objective of the present invention is to provide novel radiation-curable inner primary coating compositions which when utilized to formulate inner primary coatings on coated optical glass fibers for use in ribbon assemblies, provides the inner primary coating with the inherent property to release as a coherent layer from an optical glass fiber during ribbon stripping to provide a clean, bare optical glass fiber.
A further objective of the present invention is to provide novel coated optical glass fibers containing an inner primary coating having the inherent property of releasing as a coherent layer from an optical glass fiber during ribbon stripping to provide clean, bare optical glass fibers.
Another objective of the present invention is to provide novel ribbon assemblies containing at least one coated optical glass fiber having an inner primary coating which has the inherent property of releasing as a coherent layer from the optical glass fiber during ribbon stripping to provide a clean, bare optical glass fiber.
Surprisingly, the above objects and other objects can be obtained by utilizing a releasing oligomer according to the present invention.
The present invention provides a novel radiation-curable oligomer having a number average molecular weight of at least about 1,500 comprising:
a polyester backbone;
a plurality of carbon-containing side chains connected to said polyester backbone, wherein said carbon-containing side chains comprise at least about 5% of number average molecular weight of said oligomer; and
at least one radiation-curable functional group bound to said polyester backbone.
Preferably, the carbon-containing side chains are located sufficiently near to the ester linkages to provide hydrolytic stability to the polyester backbone.
The present invention also provides a novel radiation-curable oligomer having a number average molecular weight of at least about 1,500 formulated from:
a hydroxy functional compound containing a radiation-curable functional group; and
a hydroxy functional carboxylic acid which is branched and/or which contains a non-terminal hydroxy functional group that provides a polyester having carbon-containing side chains upon reaction of the hydroxy functional group to form an ester functional group.
The present invention also provides a novel radiation-curable oligomer formulated from the reactants according to the following formula:
Rxe2x80x94Ixe2x80x94((BHCA)sxe2x80x94Axe2x80x94(BHCA)v)wxe2x80x94Ixe2x80x94R
wherein R=a hydroxy functional compound containing a radiation-curable functional group:
I=a polyisocyanate compound:
BHCA=a hydroxy carboxylic acid which is branched and/or which contains a non-terminal hydroxy functional group that provides a carbon-containing side chain bound to the polyester backbone upon reaction of the hydroxy functional group to form an ester group; and
A=a polyol;
W=1 to about 20 on average; and
S+V=4 to about 20 on average.
The present invention further provides a novel radiation-curable inner primary coating composition adapted for forming inner primary coatings on optical glass fibers formulated from a composition comprising at least one radiation-curable releasing oligomer.
The invention also provides a novel coated optical glass fiber containing at least one inner primary coating formed from the novel radiation-curable, inner primary coating composition, and a ribbon assembly containing at least one of the coated optical glass fibers.